fix: prevent empty bottom margin when focusing input on macOS desktop (#705)

* fix: prevent empty bottom margin when focusing input on macOS desktop

Add `interactive-widget=resizes-visual` to the viewport meta tag so
Chromium only resizes the visual viewport (not the layout viewport)
when the virtual keyboard appears. This fixes the phantom margin that
showed up on macOS Electron when the text input received focus.

Closes chatboxai/chatbox#2023

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* fix: remove mobile-only viewport params from desktop build

Remove `height=device-height` and `viewport-fit=cover` from index.ejs
(Electron desktop build) as they are mobile-specific and trigger
Chromium's Virtual Keyboard API on macOS, causing an empty bottom
margin when focusing the input field.

index.html (Web/Mobile) keeps these params as they are needed there.
Also reverts the `interactive-widget=resizes-visual` approach due to
poor browser compatibility.

Fixes chatboxai/chatbox#2023

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* fix: strip mobile viewport params in desktop builds via Vite plugin

Add `stripMobileViewport()` Vite plugin that removes `height=device-height`
and `viewport-fit=cover` from the viewport meta tag during desktop (Electron)
builds only. These mobile-specific params trigger Chromium's Virtual Keyboard
API on macOS, causing an empty bottom margin when focusing input fields.

Mobile and Web builds are unaffected — they keep the full viewport meta.
Also reverts the previous index.ejs change (file is unused in current build).

Fixes chatboxai/chatbox#2023

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* refactor: use regex to strip viewport params instead of hardcoded string

Replace exact string match with regex that finds the viewport meta tag
and selectively removes `height=device-height` and `viewport-fit=cover`.
More robust against future changes to the viewport content.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* refactor: use placeholder injection for viewport meta content

Replace regex-based stripping with a cleaner approach:
- index.html uses `%VIEWPORT_CONTENT%` placeholder
- `injectViewportContent()` Vite plugin replaces it at build time
  based on platform (desktop omits mobile-only viewport params)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

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Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
(cherry picked from commit 5010058a3b3c2cc813bb31f729e0c45f3ac5fc12)