Update sidebar toggle and icon

[KSemenenko]

Summary

Change the icon used to open the sidebar and ensure users actually have the ability to close the sidebar.

Source

• Category: UI & Organization
• Feedback video: https://youtu.be/IeUoMyx7dKE?t=28m21s

Acceptance criteria

• The sidebar open control uses a clearer, recognizable icon.
• Users can close the sidebar from the same workflow without getting trapped in an open state.
• The behavior works in responsive layouts without clipping or overlap.
• Add or update browser/UI test coverage for the relevant user flow.
• Solution tests pass before the issue is marked ready for user verification.

[KSemenenko]

Verified as implemented in local main. The sidebar toggle/icon update is covered by commit 4b3dccb (Sidebar), including responsive Library sidebar behavior and the touch viewport close/reopen test.

Validation completed on 2026-04-26 after the current follow-up work:

• dotnet test @./tests/dotnet-test-progress.rsp --solution ./PrompterOne.slnx: 1191/1191 passed
• dotnet publish ./src/PrompterOne.Web/PrompterOne.Web.csproj -c Release -o ./.artifacts/prompterone-publish -p:PrompterOneBuildNumber=0: passed locally, with the expected local warning that wasm-tools is not installed so publish ran without size optimizations.

Marked assigned to byskyphy for verification. Release-version comment still needs to be added after the release containing this change ships.

[byskyphy]

Left side still has no toggle

[byskyphy]

sidebar on the right side still has an arrow icon, which is not the icon used for sidebars.

[KSemenenko]

Follow-up for @byskyphy's left-sidebar and right-sidebar-icon comments is fixed locally, pending push/release.

What changed locally:

• added a visible left structure-sidebar toggle;
• changed the right metadata rail toggle from an arrow/chevron to a sidebar-panel icon;
• added tests for the left collapsed state and right sidebar icon contract.

Local implementation:

• Commit: 3e8a0f9 Fix reader controls and editor sidebars (Add live speed dial alongside WPM configuration #24 Update sidebar toggle and icon #29)
• Status: committed locally on main, not pushed to origin yet

Local validation after this commit:

• dotnet test @./tests/dotnet-test-progress.rsp --solution ./PrompterOne.slnx: passed, 1198/1198
• dotnet publish ./src/PrompterOne.Web/PrompterOne.Web.csproj -c Release -o ./.artifacts/prompterone-publish -p:PrompterOneBuildNumber=0: passed

Not ready for release-version review comment until the commit is pushed and shipped.