fix(tabs): handle ctrl/shift+click on links, clear address bar on new tab

[countgitmick]

Addresses the modifier-click + tab-navigation bug cluster reported by @raindropsfromsky. #113 was already closed as a duplicate of #112.

Root cause

Ctrl/Shift+click on internal links was never handled by the tab layer. Vue Router's <router-link> ignores modifier clicks, so they fell through to the main-process window-open handler and spawned a new window. Only middle-click had been wired up to open a tab.

Changes

• Ctrl/Cmd+click on an internal link opens a new background tab (the same path middle-click already used). macOS uses Cmd, leaving Ctrl+click as the secondary click. ([Bug]: when I CTRL+Click on a video that is listed in FreeTubePlusTabs, it is launched in a separate window, not in a new tab in the same window. #112, [Bug]: When I CTRL+Click on a video link, it should open in a new tab in the same window (not in a new window) #113)
• Shift+click opens a new window containing just that page. Deep-linked windows are now ephemeral: they neither restore nor persist the shared tab session, so they no longer clone the previous session or overwrite it. ([Bug]: When I SHFT+Click on a video link, it should open in a new window. Instead, the app opens a new window without the video that I launched! #116)
• Address bar syncs to the active tab, clearing on a fresh tab and showing the query on a search tab. ([Bug]: When new tab is launched, the address bar is not cleared. (Its content is common across all tabs.) #111)
• Factored a shared parseInternalLinkRoute helper used by the middle/ctrl/shift click paths; updated DESIGN-TABS.md.

Verification

ESLint clean. Each modifier-click path traced and store getters/mutations confirmed. Not runtime click-tested (GUI interaction).

Fixes #111
Fixes #112
Fixes #116