feat(x): Files panel for chats, in-app PDF viewing, file card redesign

[Gagancreates]

What

Claude-style "Files" experience for chats: see every file/doc the agent created in the current chat at a glance, open PDFs inside Rowboat, and cleaner file cards.

Files panel

• lib/session-files.ts — pure derivation of the chat's files from two sources: completed file-writing tool calls (file-writeText / editText / copy / rename follows the file / remove drops it / code_agent_run changedFiles anchored to cwd), and ```filepath fences in assistant messages (the same fences that render inline cards — this is how shell-generated files like PDFs surface). Deduped by path; tmp/ scratch files filtered.
• components/chat-files-panel.tsx — right-hand panel (mirrors version-history-panel's shell) listing each file as a FilePathCard.
• Header button — a Files icon with a count badge appears in the chat header once the chat has ≥1 file. Wired in both the full-screen chat (side-by-side panel) and the side-pane chat (full-pane overlay — the pane is too narrow for a split). Shared open/close state.

In-app PDF viewing

• Clicking Open on a PDF card routes to the existing file-view flow (file tab + PdfFileViewer, chat docks to the side pane) instead of the OS viewer.
• PdfFileViewer now also handles absolute/~ paths (e.g. agent-saved Desktop PDFs) by loading through shell:readFileBase64 into a blob URL — the app://workspace protocol is boundary-guarded and can't serve them. Files over the 10MB IPC cap fall back to the existing "Open in system" state.
• Fixed a navigateToView no-op: re-navigating to the already-open file from a maximized chat pane now un-maximizes so the file becomes visible.

File card redesign

• FilePathCard gets a document-page glyph (folded corner, extension label color-accented by type) instead of generic lucide icons, tighter typography, hover polish, and a hover-revealed "Reveal in Finder" action. Full-width as before.

Notes

• Renderer-only; no IPC or schema changes. npm run typecheck passes.
• Known gap: files written by sub-agents (spawn-agent child turns) aren't in the parent conversation, so they don't appear in the panel yet — needs per-child-turn transcript fetches as a follow-up.

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