fix(attachments): preserve extensions when shortening filenames

[jonathanKingston]

Long video and archive filenames lose their extension when storage truncates the whole name to 80 characters, causing valid attachments to be rejected as unsupported. Shorten the sanitized stem while reserving space for the extension. Regression tests attach long MOV and ZIP names, verify the bounded storage filename, and read the stored bytes back.

Validation: pnpm run check; focused regressions were verified to fail before the fix and pass afterward. These changes affect data/tool operations with unchanged DOM and layout.

After the full check, rebased onto updated main and repeated typechecking plus the focused regression suite on the rebased code.

[github-actions]

🖥️ PR preview

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  _{Static demo of the renderer against a mock API — the browser pane, terminal, and all main-process IPC are stubbed, so it never proves main-process behaviour. Rebuilt whenever a push changes the demo bundle; these links go live once the queued deploy lands, shortly after this comment. Removed when the PR closes.}

[copse-release-bot]

Reference screenshots ready for review

Review GitHub’s image diffs in screenshot PR #2619.
Merge it (or enable auto-merge) to apply the accepted PNGs to codex/preserve-long-attachment-extensions.

Rendered for cae85d89e0a9 by CI run 34304181899; the immutable artifact is reference-screenshot-candidates-34304181899.

If this source branch moves, a later successful render closes the stale review PR and replaces this link.