fix(email): preserve reader folder for message actions

[adabarbulescu]

Summary

Reader-level email actions now keep using the opened message's original folder/account instead of the currently selected
library folder, so permanent delete and related actions address the correct IMAP mailbox. Destructive reader actions also
check backend {success:false} responses before removing the email from the UI, preventing failed deletes from looking
successful.

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 #5569

Type of Change

• Bug fix (non-breaking — fixes a confirmed issue)

Checklist

• I searched open issues and [open PRs](https://github.com/
  pewdiepie-archdaemon/odysseus/pulls) — 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

1. Configure an account with multiple IMAP folders.
2. Open a message from a folder other than the currently selected Library folder.
3. Switch the selected folder while keeping the reader open.
4. From the reader, test mark read/unread, flag, archive, move to spam,
   delete, and delete permanently.
5. Confirm each action targets the folder/account from which the message
   was originally opened.
6. Simulate or trigger a backend {success:false} response and confirm
   destructive actions leave the message visible and show an error toast.

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

• Screenshot or short clip of the change in the running app, attached below. Mobile screenshot too if the change
  • 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.
• No new component patterns. If a similar widget already exists in the app, extend it instead of writing a parallel
  one.

Screenshots / clips

N/A: no visual rendering change. This only changes which folder/account reader actions send to the existing API endpoints and how failed responses are handled.