feat(webui): add chat composer collapse toggle, dynamic theme color mixing, and mobile canvas rail responsiveness

[mdnaimul22]

Summary

This PR enhances the WebUI experience by introducing a chat composer collapse/expand toggle, dynamic theme highlight color mixing, and responsive mobile canvas rail positioning—all built with clean CSS specificity and zero !important anti-patterns.

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Key Features & Improvements

1. 🎛️ Chat Composer Collapse / Expand Toggle

• Added a sleek chevron toggle button in #progress-bar-box allowing users to collapse/expand the bottom controls and extensions toolbar.
• Added reactive state management (composerCollapsed, toggleComposerCollapsed()) in Alpine.js input-store.js.
• Clean CSS encapsulation that hides input accessories when collapsed without breaking plugin extension hooks.

2. 🎨 Dynamic Theme Highlight & Color Mixing

• Introduced --color-highlight in light and dark theme definitions in index.css.
• Upgraded the Send button and action states in chat-bar-input.html to use modern color-mix(in srgb, var(--color-highlight) ...) for smooth hover and active state transitions instead of hardcoded hex values.
• Standardized .chat-button to 2.5rem and its internal SVG icon to 1.3rem for optimal touch-target balance.

3. 📱 Mobile Right Canvas Rail Alignment & Responsive Breakpoints

• Aligned mobile .right-canvas-rail with the upstream top: 33%; transform: translateY(-50%) proportion, ensuring perfect upper-third positioning across mobile phones, tablets, and vertical/portrait monitors.
• Standardized mobile media queries to the framework-standard 480px breakpoint.

4. 🧹 Code Cleanliness & Zero !important Flags

• Removed all !important overrides across progress.html, right-canvas.css, and chat-bar-input.html, relying strictly on natural CSS specificity and stylesheet cascade.
• Kept Git diff 100% surgical with original selector spacing intact.

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Changed Files

• webui/components/chat/input/input-store.js — Composer collapse state.
• webui/components/chat/input/progress.html — Collapse toggle button and clean collapsed styling.
• webui/components/chat/input/chat-bar-input.html — Dynamic color mixing and rem dimensioning.
• webui/components/chat/input/bottom-actions-bar.html — Actions bar responsiveness.
• webui/components/canvas/right-canvas.css — Standardized 480px breakpoint and 33% vertical alignment.
• webui/index.css — --color-highlight theme token.

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Checklist

• Tested across desktop and mobile viewports.
• Verified collapse/expand toggle state persistence.