feat: add global search for findings and reports (#1981)

[SathvikaSingoti]

Description

Adds a global search feature for findings and reports, accessible from a search bar in the top navigation (both desktop and mobile).

Backend: New GET /api/v1/search?q={query}&limit={limit} endpoint in routes.py. Searches the caller's findings (by title/description) and reports (by name) using SQLite LIKE, scoped to owner_id per the existing BOLA-prevention pattern (issue #401) used throughout this file. User input is escaped against %/_/\ before being used in the LIKE pattern so literal wildcard characters in a search query can't produce unintended matches.

Frontend: New GlobalSearch component wired into AppShell's desktop top bar and mobile header. Input is debounced (300ms, via a new reusable useDebouncedValue hook) before calling the backend, and results render in a dropdown grouped by Findings/Reports. Clicking a result navigates to the relevant list page (Findings or Reports), no per-item detail route exists yet in the app, so this is the closest correct behavior today.

Restructured AppShell.tsx to add a shared top bar row (search) above <main> for desktop, while preserving the existing sidebar-offset layout behavior.Also documents the new endpoint in docs/API.md under a new "Search API" section, matching the existing per-endpoint format.

Related Issues

Closes #1981

Type of Change

• New feature (non-breaking change which adds functionality)

How Has This Been Tested?

• Added testing/backend/integration/test_search.py - 10 tests covering: missing/invalid query params, matching by finding title, matching by finding description, matching by report name, case-insensitivity, empty-result handling, LIKE wildcard escaping (%/_ treated as literal characters, not SQL wildcards), limit bounds enforcement, and owner-scoping (a finding belonging to another owner never appears in results).
• Added frontend/testing/unit/components/GlobalSearch.test.tsx - covers initial render, debounce behavior (no call on empty query, call after typing), result rendering, no-results state, error state, navigation on result click, and closing the dropdown on Escape.
• Ran ./testing/test_python.sh - new test file: 10/10 passing.
• Ran npm run typecheck - clean.
• Ran npm run test - 546/546 passing (including the two pre-existing AppShell.test.tsx sidebar-width tests, which needed the --sidebar-width CSS var re-applied directly to <main> after the layout restructure).
• Ran npm run build - builds clean.
• Ran bash scripts/check-artifacts.sh origin/main - no tracked generated artifacts or cache files in the diff.
• Manually verified: typing in the search bar returns matching findings/reports, clicking a result navigates and clears the input, Escape closes the dropdown, clicking outside closes the dropdown.

Checklist

• My code follows the code style of this project.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• I have made corresponding changes to the documentation.
• My changes generate no new warnings.