feat(file-preview): drag-range selection + right-click handoff to terminals & agent

[iptoux]

Summary

Extends the CodeView file preview with drag-based line range selection and a right-click context menu that ships the highlighted snippet to any terminal in any workspace, attaches it to any workspace's BLXCode Agent context, or copies it to the clipboard. Cross-workspace handoffs prefix the snippet header with the source workspace label so the receiving agent can disambiguate.

Highlights

• Drag selection in code_view.rs: mousedown → mousemove extends selected: Option<(usize, usize)>; a window-level mouseup listener (installed once, cleaned up via on_cleanup) ends drags even when the cursor leaves the gutter. Single-click toggle preserved. .code-view got user-select: none.
• Right-click context menu (new module file_preview/code_context_menu.rs) with four sections, all grouped per workspace:
  • Snippet → Insert into terminal — fenced markdown block written verbatim via pty_write (base64).
  • Full context block → Insert into terminal — same snippet wrapped in the standard ⟪ BLXCode attached context ⟫ envelope.
  • Snippet → Attach to agent — upsert_workspace_agent_context with new AgentContextKind::FileSnippet carrying inline content.
  • Clipboard — copy snippet / line range / raw text via navigator.clipboard.writeText with toast feedback.
• Cross-workspace support: new helper list_terminal_targets_all_workspaces enumerates every workspace's live terminals (shell workspaces filtered via existing is_shell_workspace), with the preview's own workspace pinned to the top and tagged with a localized current badge.
• Snippet builders: build_file_snippet_block in util.rs (clamped + UTF-8 safe, 6 unit tests) and render_file_snippet_envelope in agent_context_handoff.rs (2 unit tests covering same-workspace and cross-workspace headers).
• Wire protocol: AgentContextItem gained an optional content: Option<String> field on both src/agent_wire.rs and src-tauri/src/agent/protocol.rs; backend render_context_prompt partitions FileSnippet items into a dedicated `Attached file snippets` prompt section; render_agent_context_block mirrors this with an `## Attached file snippets` section.
• i18n: 22 new keys (CodeViewMenu*, CodeViewToast*) with real translations in all 13 locales (en, de, fr, es, it, pt_br, pl, hu, ru, ja, ko, zh_cn, zh_tw) using `{workspace}`, `{terminal}`, `{slot}`, `{agent}`, `{error}` placeholders.
• CSS: .code-context-menu themed via existing tokens (sections, workspace groups, slot sublists, accent-pill badge, themed hover via --overlay-2).
• Docs: docs/user/file-preview.md and CHANGELOG.md updated; plan tracked in .agents/plans/file-browser-rich-preview.md + .cursor/plans/code-preview-drag-select-handoff_f50216bd.plan.md.

Test plan

• `cargo check -p blxcode-ui --target wasm32-unknown-unknown` clean
• `cargo check -p blxcode` clean
• `cargo test --workspace` — 53 tests pass, including 8 new ones:
  • 6 × build_file_snippet_block_* (single/range/cross-workspace/no-lang/UTF-8/clamping)
  • 2 × render_file_snippet_envelope_* (in-workspace + cross-workspace source marker)
  • 3 × file_snippet_* constructors + render section
• Manual verification with `cargo tauri dev`:
  • Drag a multi-line range; verify continuous highlight across drag direction (both up & down)
  • Right-click inside vs outside current selection (re-selects single line when outside)
  • Insert snippet → same-workspace terminal and cross-workspace terminal; verify `source workspace:` header appears only on the cross case
  • Insert full envelope → cross-workspace terminal; verify Session block lists target workspace, snippet block lists source workspace
  • Attach to agent → preview workspace and a different workspace; verify Agent panel context list shows `Snippet · path:start-end`
  • Copy snippet/range/raw; verify toast appears in every locale
  • Right-click in `.txt` / `.log` (Text variant) — menu works without syntax highlighting
  • Switch active locale to each of the 13 languages — every section header, group label, slot label, and toast renders in that language
  • `Escape` and outside-click both close the menu

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