feat(agents): per-mode thinking effort, wider columns, drag-region fixes

[aletc1]

• Per-mode Thinking Effort: new defaultPlan/Agent/ReviewModeThinkingAtom applied alongside the model default via applyModeDefaultModel, so switching mode (or /review) also swaps thinking effort. Removes the global Thinking Effort row from Settings; keeps per-subChat overrides intact.
• Settings Preferences redesign: drop "Mode" from model labels; collapse the six model/thinking rows into three "Default Plan/Agent/Review" rows with the model and thinking selectors side-by-side on the right. Widen the settings container from max-w-2xl to max-w-5xl.
• Chat column: bump max-w-4xl -> max-w-5xl on messages, user-questions, queue/status cards, chat input, disabled input fallback, and the new-chat form so every view uses the same wider column.
• Drag-region fixes: add WebkitAppRegion: "no-drag" to interactive elements sitting under the 32px draggable top strip that were silently becoming drag regions. Affects right-side chat header buttons (Fork Locally, Open Preview, View Details, Open Terminal, Restore), terminal sidebar (Close + TerminalModeSwitcher), details sidebar, agent-plan sidebar, agent-preview desktop header, file-viewer sidebar (both headers), and expanded-widget sidebar.