feat(summary): custom summary template editor with JSON authoring

[karan68]

Summary

Adds a custom summary template editor so users can control how meeting records are
structured, including JSON-level authoring. Templates were already JSON-backed and
override-able on disk; this exposes create/edit/duplicate/delete in the UI and adds raw
JSON editing, import, and export. Implements the actionable part of #257 (editable
summarization) without a raw system-prompt override.

The fixed prompt preamble (multi-language contract + prompt-injection guard) and
api_process_transcript's signature are unchanged — only the template's section
instructions (the safe, functional part of the prompt) are editable.

What changed

Backend (frontend/src-tauri/src/summary/)

• template_commands.rs: TemplateInfo now carries source/editable/deletable; new
  api_get_template_content, api_save_template, api_delete_template (registered in lib.rs).
• templates/loader.rs: template_source, is_valid_template_id, and
  save_custom_template/delete_custom_template over the existing JSON-file store, with a
  crash-safe temp-file + backup swap on save.
• templates/types.rs: validation bounds — trim/whitespace rejection, unique section
  titles, ≤30 sections, and character limits (name 120, description 500, title 120,
  instruction 2000, item_format 500).

Frontend

• New components/SummaryTemplateManager.tsx card in Settings → Summary: list with
  Built-in/Custom badges; create / edit / duplicate / delete (delete via ConfirmationModal);
  Form / JSON toggle with raw JSON editing + Validate; Import JSON; Copy JSON.
  Section reorder (up/down), discard-changes confirmation, collision-free ids.
• New lib/summary-template.ts helpers (slugifyTemplateId, createUniqueTemplateId,
  moveArrayItem) + tests/lib/summary-template.test.mjs.
• Wired into components/SummaryModelSettings.tsx.

Docs

• README.md: "Custom summary templates" section with the schema, a full example, a
  minimal example, a field reference (with limits), and on-disk locations.

Editing model

• Editing a built-in saves a same-id custom override; deleting the override restores the
  built-in. Only custom templates are deletable.
• Freedom is within the fixed schema (format ∈ paragraph | list | string); there is
  intentionally no arbitrary system-prompt override.

Testing

• Backend: 17/17 summary::templates tests pass (incl. new validation + atomic-save tests).
• Frontend: tsc --noEmit clean; 66/66 Bun tests (incl. 3 new helper tests).
• Manual (recommended before release): create/edit/duplicate/delete; Form↔JSON round-trip;
  Import/Copy JSON; edit a built-in → becomes Custom → delete reverts; invalid JSON/format
  is rejected with a clear message; template appears in the meeting summary picker.

Notes

• No new dependencies. Clipboard uses navigator.clipboard (already used elsewhere in the app).
• FUTURE_ROADMAPS.md (living project roadmap) is included in this push.