feat: add in-cell nullify button for values and sections

[bosiakov]

Helm overlays often need to set a key to explicit null (key: ~) so the chart's default container drops out of the rendered manifest. Until now users had to hand-edit the file. This adds a one-click affordance.

• ui/widget/nullify_icon.go: inline tilde button rendered on row hover or when the cell is already nullified
• ui/widget/null_pill.go: pill chip that replaces the editor when a cell is null or sits below a nullified ancestor section
• ui/page/values_controller_actions.go: onCellNullify handler that toggles the flag, severs participating anchors and aliases, and republishes the column yaml
• ui/state/values_state.go: NullifiedKeys map on CustomColumnState with MarkNullified, IsNullifiedDirect, IsNullifiedCovered, ClearNullified
• ui/state/editor_state.go: collectOverrides emits flagged keys as Value=tilde, Type=null, drops every descendant of a nullified section, and sorts orphan keys for byte-stable output
• ui/page/values_controller.go: on file load, seed NullifiedKeys from any Type=null leaves so saved overlays round-trip
• service/values_types.go: IsNullLeaf predicate on FlatValueEntry
• service/values_service.go: export TypeNull so the literal lives in one place across service, state, page, and widget packages
• ui/widget/override_table.go: layoutNullifyButtonSlot keeps a stable footprint whether the button is visible, hidden, or replaced by the pill, so badge position does not shift on hover
• ui/widget/override_table_cells.go: bool switch cell pads its returned width to the flex slot so the trailing badge and button stay pinned to the right
• ui/widget/override_table_events.go: typing into a covered descendant clears the ancestor null flag so keystrokes are not silently dropped at save
• ui/widget/paint_primitives.go: paintHairlineBorder consolidates the four edge rect border idiom previously open coded in null_pill, nullify_icon, extras_filter_pill, and button