feat: Refactor file attachments, attachment previews and external file support

[charlesvien]

• File attachments refactor - Refactors attachment support. Adds AttachmentsBar component for previewing attachments (you can also unattach them). Refactors useTiptapEditor and useDraftSync to handle image attachments alongside text mentions.
• External file support - Adds readAbsoluteFile tRPC endpoint so files outside the repo can be read and displayed in panels. Updates prompt-builder.ts to detect absolute paths and route them to the correct endpoint. Fixes path mangling in usePanelLayoutHooks.tsx where absolute paths were incorrectly prepended with the repo path.
• Image preview in code editor - CodeEditorPanel now detects image files and renders them as elements instead of showing binary blob in the code editor.
• Panel improvements - PanelMessage gains a detail prop for secondary text. Error states now show the file path for easier debugging.

[charlesvien]

• feat: Refactor file attachments, attachment previews and external file support #939 👈 (View in Graphite)
• feat: Improve session error handling and display #928
• feat: Add pin button back to task hover toolbar #934
• feat: Add ability to dismiss individual queued messages #927
• fix: Fix custom answer in the question prompt not working #923
• feat: Implement bounce dock icon notification setting #922
• fix: Start preview session without waiting for repo selection #921
• fix: Ensure .claude/settings.local.json exists for permission persistence #920
• main

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