feat: update sidebar design

[sehyod]

This adds a generic Sidebar component, uses it for the left sidebar, and replaces the right panel logic with a second sidebar on the right
This also fixes the background of the splashscreen to be the same as the one of the welcome screen, instead of white

NB: This PR does not update the sidebar panels (like the references panel, the file explorer panel, etc.)

Closes #371

[codecov]

Codecov Report

Merging #383 (20b4ea5) into main (a1c7f9e) will decrease coverage by 1.08%.
Report is 1 commits behind head on main.
The diff coverage is 36.65%.

@@            Coverage Diff             @@
##             main     #383      +/-   ##
==========================================
- Coverage   84.16%   83.09%   -1.08%     
==========================================
  Files         175      161      -14     
  Lines       10016     9192     -824     
  Branches     1101     1106       +5     
==========================================
- Hits         8430     7638     -792     
+ Misses       1575     1540      -35     
- Partials       11       14       +3     

Files Changed	Coverage Δ
src/application/App.tsx	0.00% <0.00%> (ø)
src/application/sidebar/LeftSidePanelWrapper.tsx	0.00% <0.00%> (ø)
src/application/sidebar/RightSidePanelWrapper.tsx	0.00% <0.00%> (ø)
src/application/sidebar/icons.tsx	0.00% <0.00%> (ø)
src/components/VerticalResizeHandle.tsx	0.00% <0.00%> (ø)
src/splash.tsx	0.00% <0.00%> (ø)
src/application/components/SideBar.tsx	100.00% <100.00%> (ø)
src/features/ai/sidebar/ChatPanel.tsx	100.00% <100.00%> (ø)
src/features/ai/sidebar/RewriterPanel.tsx	100.00% <100.00%> (ø)

... and 16 files with indirect coverage changes

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