fix(tui): pluralize sessions command category

[OnlyYu1996]

Fixes PlatformNetwork/bounty-challenge#53114.

Changes:

• Uses Sessions for command-category display names in the TUI command system.
• Updates the command modal and command card category labels to match palette session grouping.
• Leaves singular Session names intact where they refer to one specific view/session, not a grouping category.
• Updates regression assertions for the affected category labels.

Validation:

• rustfmt +stable --edition 2024 --check src/cortex-tui/src/commands/types.rs src/cortex-tui/src/modal/commands.rs src/cortex-tui/src/cards/commands.rs
• cargo +stable test --package cortex-tui test_command_category_display_name -- --nocapture
• cargo +stable check --package cortex-tui --lib
• git diff --check

Note: cargo +stable fmt --package cortex-tui --check is blocked on current main by unrelated pre-existing formatting diffs in src/cortex-common/src/lib.rs and src/cortex-tui/src/question.rs; the files changed here pass rustfmt directly.

[greptile-apps]

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