feat: add application navigation

[darwin67]

Summary

• add a responsive application sidebar with organization identity and a scrollable workspace list
• move organization destinations into a compact dropdown and keep account controls in the header
• add an authenticated organization chooser and remove duplicate page-local navigation
• improve mobile navigation accessibility with a native modal dialog, focus restoration, expanded state, and scroll locking
• constrain organization menus for short viewports and identify active destinations

Regression coverage

• verifies the mobile drawer uses an explicit modal dialog contract rather than a checkbox
• verifies organization menus remain scrollable in constrained viewports
• verifies the active organization destination exposes aria-current="page"

Verification

• mix test test/textbin_web/live/ui/organization_live_test.exs (17 passed)
• focused organization/workspace/paste LiveView suite (55 passed)
• mix precommit (427 passed)
• mix assets.build
• git diff --check