fix: seamless fullscreen transition for custom and native window decorations

[SubmitCodes]

Fixes #5

Summary

Fixes the fullscreen transition issues on Windows when toggling fullscreen from a maximized state with custom (frameless) window decorations.

Problem

• Taskbar clipping: When running with custom decorations (decorations: false), toggling fullscreen while the window was maximized caused the window bounds to be clamped to the Windows work area, leaving a gap/bar over the taskbar at the bottom.
• Window jump / stutter: Calling DOM requestFullscreen() or chaining unmaximize() -> setFullscreen(true) in JavaScript sent back-to-back resize events and triggered Windows DWM restore animations, causing the window to visibly jump, shrink, and drop frames.

Changes

• Native Backend Fullscreen Commands: Implemented set_fullscreen, toggle_fullscreen, and is_fullscreen commands in Tauri Rust backend (commands.rs).
• Direct 1-Step Expansion (Windows): When entering fullscreen from maximized with custom decorations, clears WS_MAXIMIZE in memory and resizes directly to full monitor bounds in one single frame via SetWindowPos, eliminating double-resizes and DWM warping.
• Maximized State Restoration: Seamlessly restores the maximized window state upon exiting fullscreen if the window was previously maximized.
• Native Decorations Isolation: Preserves 100% native OS window animations when nativeDecorations is enabled by bypassing custom logic and calling native fullscreen directly.
• Synchronized Composable (useFullscreen.ts): Manages a shared reactive isFullscreen ref across shell.vue, bubble-nav.vue, and index.vue, with fallback to @vueuse/core on web.

Verification

• Tested toggling fullscreen from maximized state with custom decorations (covers entire screen & taskbar smoothly with zero jump).
• Tested toggling fullscreen from normal/windowed state with custom decorations.
• Tested toggling fullscreen with native decorations enabled.
• Verified clean typecheck and cargo build across the workspace.