feat: add Tabler icon library across SPA

[pskillen]

Problem

The SPA has no icon library. Navigation and actions use Unicode arrows (←, →) and a single emoji cog (⚙) in map controls. This is inconsistent, poor for accessibility, and doesn't match Mantine 9's visual language.

Intended outcome

Add @tabler/icons-react (Mantine's native icon set) and roll icons through the SPA tastefully — navigation, primary CRUD actions, import/export, and map settings. Data-dense surfaces (tables, badges, frequency text) stay text-first.

Tasteful-use rules

• NavLink / Button leftSection with label kept
• Standard action vocabulary (plus, pencil, trash, arrow-left, download, upload)
• Size 16 in nav/links, 18 in ActionIcon; stroke={1.5}
• Named imports per file; no barrel re-exports
• Icon-only controls must have aria-label

Slices

1. Foundation — dependency, iconSizes.ts, display conventions
2. Shell navigation — App.tsx, reference index, ActiveProjectBar
3. CRUD and back navigation — list/detail/edit routes, ConfirmDeleteModal, ZoneMemberPicker
4. Import/export/workflows — ImportDropzone, Export, SummaryCard, ProjectList
5. Map and location — MapControls, UseMyLocationButton
6. Docs and PR

Affected

• package.json, src/lib/iconSizes.ts
• src/App.tsx, nav and action surfaces across routes/components
• docs/reference/display-conventions.md, docs/features/ui/

Out of scope

• Leaflet marker divIcon styling
• OpenGD77 CSV Icon columns
• Icons on table rows, BandPill, mode badges
• Alternative icon libraries

Workflow

Branch {num}/paddy/icon-library from origin/main, atomic conventional commits per slice, PR linking Closes #{num}.

[pskillen]

Execution plan: .cursor/plans/icon_library_rollout_f91bef48.plan.md

Progress tracking:

• docs/features/ui/icons-progress.md
• docs/features/ui/icons-outstanding.md

Branch: 64/paddy/icon-library