fix(mobile): don't open the sidebar drawer while selecting message text

[BrunooMoniz]

Summary

On mobile, dragging to extend a text selection inside a chat message opened the left sidebar drawer. The swipe-to-open gesture in _initChatSwipeToOpenSidebar (static/js/sidebar-layout.js) armed for any one-finger horizontal drag that started anywhere over #chat-container, so growing a selection to copy a reply was captured as an edge-swipe and popped the drawer open over the text you were selecting. This PR gates the gesture so it arms only from a narrow 24px strip at the screen's left/right edge and bails whenever a text selection is active at touch start or appears/grows during the drag. A drag that starts over a message body is now left entirely to native text selection, while a genuine edge-swipe still opens the drawer on either side and the hamburger button and backdrop-tap close path are unchanged. The gesture is already mobile-only (gated to viewports narrower than 768px), so desktop is unaffected.

Target branch

• This PR targets dev, not main. All PRs land in dev; main is curated by the maintainer at each release. If your PR is on main by accident, click "Edit" on this PR and change the base.

Linked Issue

Fixes #5584

Type of Change

• Bug fix (non-breaking — fixes a confirmed issue)
• New feature (non-breaking — adds new behaviour)
• Breaking change (changes or removes existing behaviour)
• Refactor / cleanup (behaviour unchanged)
• Documentation only
• CI / tooling / configuration

Checklist

• I searched open issues and open PRs — this is not a duplicate.
• This PR targets dev
• My changes are limited to the scope described above — no unrelated refactors or whitespace changes mixed in.
• I actually ran the app (docker compose up or uvicorn app:app) and verified the change works end-to-end. Type-checks and unit tests are not enough.

How to Test

Steps a reviewer can reproduce (full app):

1. docker compose up -d --build, open the chat on a phone (or in DevTools with mobile/touch emulation and the viewport narrower than 768px).
2. Send any prompt, wait for a reply, then press on the reply text and drag horizontally across it to select it (as you would to copy it).
3. On current dev: the left sidebar drawer slides open over the message and the selection is interrupted. With this branch: the text selection extends normally and the drawer stays closed. A deliberate swipe from the very left/right screen edge still opens the drawer, and the hamburger button and backdrop tap still open/close it.

What was verified for this change:

• The behaviour was confirmed in a mobile browser (WebKit, 390px) — current dev (before, drag-to-select opens the drawer) vs this branch (after, the selection stays and the drawer stays closed). See the screenshots below.
• node --check static/js/sidebar-layout.js — passes.
• python -m pytest tests/test_sidebar_swipe_js.py — 5 passed. The test loads the real module under Node and dispatches synthetic touch events: a mid-content selection drag, an edge-start drag with a live selection, and a selection that grows mid-drag all keep the drawer closed; a genuine left-edge and right-edge swipe both still open it. The three selection cases fail on current dev (the drawer opens) and pass on this branch, so the test pins the fix.
• The change is limited to one client-side module plus its new test, so the rest of the pytest suite is unaffected.

Files changed:

• static/js/sidebar-layout.js — the swipe-to-open handler now arms only from a narrow edge strip (EDGE_ZONE) and bails on an active or growing text selection (hasTextSelection); no styling, markup, or other behaviour changes.
• tests/test_sidebar_swipe_js.py — a Node-driven regression test (same subprocess pattern as tests/test_markdown_rendering_js.py) that dispatches synthetic touch events against the real module.

Visual / UI changes — REQUIRED if you touched anything that renders

Anything that changes what the UI looks like — buttons, icons, padding, colors, fonts, spacing, layout, CSS, HTML, SVG, or any static/js/ module that draws to the DOM — needs all of the following.

• Screenshot or short clip of the change in the running app, attached below. Mobile screenshot too if the change affects mobile.
  • How the screenshots were captured: the real static/js/sidebar-layout.js module and static/style.css rendered in WebKit at a mobile viewport (390x844) on the default dark theme, showing the drawer-open (before, current dev) vs drawer-closed with the selection intact (after, this branch). The gesture is mobile-only (gated below 768px), so there is no desktop case to show.
• Style match: the change uses Odysseus's existing visual language. Specifically:
  • Reuse existing CSS variables (--red, --fg, --bg, --card, --border, etc.) — do not introduce new color values, font sizes, or spacing units.
  • Reuse existing button/input/card/border classes. Don't invent parallel styling.
  • No Unicode emoji in UI or code. Use inline SVG (matching the monochrome icon style already in static/index.html) or plain text.
  • Monospaced font (Fira Code) for primary UI text. Don't override.
  • Dark theme is the default; any light-mode work must be wired through the existing theme system, not hard-coded.
  • This change adds no CSS, markup, colors, fonts, or components — it only guards an existing touch gesture, so the visual language is unchanged.
• No new component patterns. If a similar widget already exists in the app, extend it instead of writing a parallel one.
• I am not an LLM agent submitting a bulk PR. If you are, please open an issue describing the problem first — bulk auto-generated PRs that don't match the project's visual style are closed on sight, even when the underlying fix is correct.

Screenshots / clips

Mobile — before (current dev)

Mobile — after (this PR)