feat(cli): unify session modes in footer and stabilize Composer layout

composer-layout-spec.md

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+# Layout Refinement: Unified Session State & Composer Organization
+
+## Goal
+
+The objective is to consolidate all persistent "Session Modes" into a single,
+unified status area in the footer and reorganize the remaining transient
+elements in the Composer for a cleaner information hierarchy.
+
+## Phase 1: Unified Footer Modes
+
+The footer's first column is expanded to be the **Unified Mode Indicator**. It
+natively incorporates the three primary state toggles of the application.
+
+- **Approval Mode:** (manual, auto-accept, plan, YOLO) - Always visible.
+- **Shell Mode:** Visible only when active.
+- **Raw Markdown Mode:** Visible only when active.
+
+### Footer Layout
+
+- **Header:** `mode (Shift+Tab)`
+- **Data Row:** Multiple modes are displayed in their respective semantic
+  colors, separated by a middle dot (`·`).
+- **Example:** `plan · shell · raw`
+
+## Phase 2: Composer Cleanup & Swap
+
+With all modes moved to the footer, the Composer is simplified to handle only
+transient notifications and active processing states. These two areas are
+swapped across the horizontal divider.
+
+### 1. The "Above Divider" Zone (Environment Alerts)
+
+Reserved for transient notifications that alert the user to environment-level
+changes.
+
+- **Toast Messages:** (e.g., "Press Ctrl+C again to exit")
+- **Shortcuts Hint:** (e.g., "? for shortcuts") - Remains flush right.
+
+### 2. The "Below Divider" Zone (Active processing)
+
+Reserved exclusively for the application's current activity. It sits directly
+above the input prompt for maximum visibility during streaming.
+
+- **Loading Indicator:** (e.g., "Thinking...", "Executing Hooks")
+- **Status Display:** (Context usage summary)
+
+## Target Layout Mockup
+
+### Composer Area
+
+```text
+[ConfigInitDisplay]
+[QueuedMessageDisplay]
+[TodoTray]
+
+[ToastDisplay]                                         [ShortcutsHint]
+----------------------------------------------------------------------
+[LoadingIndicator (e.g., Thinking...)]
+                                                       [StatusDisplay]
+
+[InputPrompt]
+```
+
+### Footer Area (Status Line)
+
+```text
+ mode (Shift+Tab)        workspace                /model
+ manual · shell · raw    ~/src/gemini-cli         gemini-pro
+```
+
+## Key Principles
+
+- **Single Source of Truth:** All "modes" now live in the footer. If a user
+  wants to know what state the CLI is in, they only need to look at the far-left
+  footer item.
+- **Reduced Jitter:** Moving the Shell and Markdown indicators out of the
+  Composer reduces vertical jumping in the main interaction area.
+- **Immediate Feedback:** The Loading Indicator remains closest to the Input
+  Prompt, providing the most direct feedback during generation.