Wire the Tasks tab into nav; fold in buckets/projects/labels and recurrence

The Tasks tab (/tabs/tasks) existed server-side and was tested, but
nothing in the nav linked to it -- it was pure dead weight in the other
direction. Wiring it up as the natural home for everything that was
either misplaced in Settings or missing a web UI entirely:

- Maintenance Buckets, Projects, and Labels moved out of Settings and
  into the Tasks tab (restyled from Settings' opaque slate cards to the
  glass/backdrop-blur look already used by the tab's chain/atom cards --
  they're now embedded in index.html's page shell, not a standalone
  page, so the shared bg-card/bg-input classes from that shell apply).
  Settings keeps only what's actually settings: Passkeys, Trusted
  Agents, Data Sources.
- Added a Recurrence section to the task-detail modal (freq/interval/
  weekday form, posting to a new POST /tasks/recurrence -- the HTMX
  counterpart to the widget API's HandleWidgetTaskRecurrence). Native
  task recurrence previously had zero web UI at all, only reachable via
  the Android widget's RecurrenceEditDialog.

Also fixed a real bug found while touching this code: HandleGetTaskDetail's
source switch only had a case for "trello" -- opening any native ("doot")
task's detail modal, which is most tasks in this tab, showed a blank
title and description. Factored both call sites (initial GET and the
re-render after a recurrence edit) through one loadTaskDetailData helper
and added the missing "doot" case. Also fixed task-detail.html's styling,
which was still using pre-dark-theme classes (text-gray-900 etc.) --
functionally invisible text on the modal's dark background.

Verified with a throwaway local server (real templates + real DB, not
the MockRenderer the unit tests use) seeded with a recurring task,
a bucket, a project, and a label -- confirmed all five touched routes
render 200 with the expected content, including the populated
Buckets/Projects/Labels sections and a real weekly-recurrence form
with the correct weekdays pre-checked. Caught and fixed a copy bug
this way too ("every 2 weeklys" -> "every 2 weeks"). Not committed;
deleted after use.

go build ./..., go vet ./..., and go test ./... all clean.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EZ7ikw2ukGJFTHE3bJS7zL

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