Show session-specific file diffs in the inspector

[calvintvu]

Context

The session inspector lists edited files but does not show the changes that produced them. This adds a per-file view backed by recorded session edits instead of the current working tree. Changes remain in session order, while file writes show their full content.

Changes

• Make edited filenames clickable and open a dedicated file-changes panel over the inspector
• Show sequential edits with colored lines and addition/deletion statistics
• Preserve recorded changes from Claude, Codex, and OpenCode, including accurate OpenCode replacement patches
• Add a session-scoped /api/diff endpoint with loading, empty, and error states

Test

• go test ./cmd/coslash -count=1 — passed
• go build ./... — passed
• npm test — passed
• npm run build — passed
• Manual: not run after restarting the collector

Screenshots

• Session inspector — clickable file list and overlaid sequential diff panel

[luhe19001]

@calvintvu, please review these two findings.