feat: add 'slip into the tail' feature

[PlkMarudny]

Simple implementaion of sliding the clip being edited within in/out points (only to the left)

When a clip is selected and has in/out points set, 'slide' allows to move in/out markers of the clip simultaneously
sliding the trimmed part of the clip to the right by playhead time timelline - clip in point on the timeline. A button labelled slide has been added. It the operation is not possible, the toast is displayed.

Type of change

• Bug fix
• New feature (editing tool/GUI)
• Docs
• Refactor / internal

How was it verified?

• node --check server.js && node --check app.js && node --check mcp-server.js passes
• Opened the editor and confirmed the change in preview
• Confirmed the change in an export (fast or realtime), if it affects rendering
• Updated CLAUDE.md / README.md if the schema, props, or API changed

Checklist

• No new runtime dependencies added
• Preview and export render identically (single compositor)
• Commits are focused and messages are descriptive

Summary by CodeRabbit

• New Features

  • Added a Slide editing tool for slipping a selected clip’s source under the playhead without changing its timeline position or duration.
  • Linked audio and video clips remain synchronized during Slide edits.
  • Added a toolbar button, tooltip, and help entry for the Slide operation.
  • Displays a notification when the clip cannot slide further due to insufficient source media.

• Documentation

  • Updated editing feature and usage documentation to include Slide.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro Plus

Run ID: 4f76df41-e614-47e6-a5f7-56f9643145f8

📥 Commits

Reviewing files that changed from the base of the PR and between 863e55e and 8c02d68.

📒 Files selected for processing (1)

• CLAUDE.md

🚧 Files skipped from review as they are similar to previous changes (1)

• CLAUDE.md

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📝 Walkthrough

Walkthrough

Adds a Slide timeline operation that shifts a selected clip’s source window to the playhead while preserving timeline placement and duration, synchronizing linked AV clips, validating available media tail, and updating toolbar documentation.

Changes

Slide clip editing

Layer / File(s)	Summary
Slide semantics and documentation CLAUDE.md, README.md, CHANGELOG.md	Documents the playhead-based source slip, media-tail constraint, linked AV synchronization, and Slide support among editing operations.
Playhead-based slide operation app.js	Adds validation, source-window adjustment, undo support, linked AV synchronization, persistence, inspector refresh, and paused-preview seeking.
Slide toolbar integration index.html, app.js	Adds the Slide toolbar button, tooltip, shortcut/help entry, and click-handler wiring.

Estimated code review effort: 3 (Moderate) | ~20 minutes

Sequence Diagram(s)

sequenceDiagram
  participant User
  participant SlideToolbar
  participant slideClipAtPlayhead
  participant syncLinkedTiming
  participant PreviewMedia
  User->>SlideToolbar: Click Slide
  SlideToolbar->>slideClipAtPlayhead: Invoke selected-clip operation
  slideClipAtPlayhead->>syncLinkedTiming: Synchronize linked AV timing
  slideClipAtPlayhead->>PreviewMedia: Seek while paused

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🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title matches the new clip slide/slip editing feature, including the tail/insufficient-media behavior.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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[coderabbitai]

Actionable comments posted: 1

🤖 Prompt for all review comments with AI agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Inline comments:
In `@CLAUDE.md`:
- Around line 432-435: Update the Slide (slip) source documentation to describe
tail availability using the integrated source window returned by mediaTimeAt(),
rather than the constant-speed calculation in + duration×speed. Preserve the
guidance that start and duration remain unchanged and that the UI notifies when
no unused source tail remains.

🪄 Autofix (Beta)

Fix all unresolved CodeRabbit comments on this PR:

• Push a commit to this branch (recommended)
• Create a new PR with the fixes

---

ℹ️ Review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro Plus

Run ID: e3d4c270-cec4-4dbf-93df-69c5d7dd39a4

📥 Commits

Reviewing files that changed from the base of the PR and between 4b7d8f5 and 863e55e.

📒 Files selected for processing (5)

• CHANGELOG.md
• CLAUDE.md
• README.md
• app.js
• index.html