Add category and tag management on a new Labels tab

[LouisBenjamin]

Summary

Categories already existed as a table (icon column + all) but were read-only in the app, and expense rows showed a hardcoded receipt icon regardless of category. Tags — a keyword that auto-assigns a category, e.g. "COSTCO" → Groceries — are exactly the merchant_rules the statement importer already had, just without a home outside the import flow.

• Labels tab: new fourth tab on the group screen (Expenses / Balances / Members / Labels) with full CRUD for categories and tags. Global categories render read-only with a "Default" chip; group categories are editable/deletable.
• Tags = merchant_rules: no new table — the Labels tab is a second UI over the same data statement import has always used. MerchantRulesScreen is retired; /import/rules now redirects to Labels.
• Migration 0012_categories.sql: category FKs go ON DELETE SET NULL (a category can now be deleted without an FK violation or orphaning data), unique category names per group, and the 10 global categories are re-seeded with Phosphor icon slugs.
• lib/core/icons.dart: the app's new icon vocabulary, built on phosphor_flutter (MIT). kCategoryIcons backs the category picker, _legacyIconAliases keeps pre-migration Material icon names resolving, categoryTint/onCategoryTint give each category a deterministic colour with no colour picker needed, and AppIcons holds semantic constants for app chrome. Every Icons.* call site in lib/ moved to it — confirmed via flutter build web --tree-shake-icons.
• Expense rows now show the category's icon (on its tint) and name, instead of a fixed receipt icon for every row.
• Add-expense autofill: new matchTagCategory runs the same tag-matching precedence import review uses, live as you type a description, and never overrides a category you picked by hand.
• Docs: updated docs/statement-import.md, added docs/categories-and-tags.md, updated the README project-structure listing.

Note: this branches off main (commit 89f8693), independent of the in-flight docs/readme-statement-import PR — the two touch overlapping README sections and whichever merges second will need a small rebase.

Testing

• flutter analyze — clean
• flutter test — 175/175 passing (updated screens_smoke_test.dart for the 4th tab, retargeted MerchantRulesScreen tests to LabelsScreen, added matchTagCategory unit tests and a new labels_tab_test.dart)
• flutter build web --tree-shake-icons — succeeds, Phosphor + Material fonts tree-shake correctly

Follow-ups (out of scope here)

• add_recurring_screen.dart doesn't get the tag-autofill treatment yet, to keep this diff reviewable.
• No colour picker — category avatars use a deterministic tint derived from the category id.

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