fix(web): align legacy thread metadata columns - #99

The Legacy Sidebar environment icons were not actually aligned in scanning columns. Relative timestamp labels widened a flex container, so the worktree and environment icons shifted horizontally from row to row.\n\nThis replaces that flexible metadata area with fixed worktree, environment, and timestamp tracks. Empty worktree and local-icon cells remain mounted, while optional port and terminal indicators extend to the left without moving the scanning columns. The newer Show worktree indicators setting is preserved inside the fixed worktree track.\n\nVisual QA on the isolated shared web app:\n- authenticated app loaded successfully through vp run dev --share after rebasing onto PR #102\n- all 10 real Legacy Sidebar rows used identical worktree, environment, and timestamp coordinates\n- labels 2m ago, 42m ago, 1d ago, 17h ago, and just now left every column unchanged\n- local icon off left an aligned empty environment slot\n- local icon on filled that same slot without moving the worktree or timestamp columns\n- worktree icons off left an aligned empty worktree slot without moving environment icons or timestamps\n- original isolated settings were restored after testing\n\nValidation:\n- focused environment-icon tests: 10 passed\n- web typecheck: passed\n- LastCode quick local CI: passed on rebased head 59ade17\n- web tests: 2,856 passed\n- server tests: 2,838 passed, 7 skipped\n- all remaining workspace suites passed\n\nThe collaborative preview screenshot operation itself failed, so there is no after image attached; visual layout was verified from the rendered browser geometry rather than inferred from source.\n\nImplemented with GPT-5.6 Codex in the T3 Code harness.