[Features] Add missing color tokens to Theme struct

[felipeospina21]

Description

The current style.Theme struct has 30 color tokens but is missing tokens for several UI elements, forcing consumers to either hardcode colors or reuse semantically wrong tokens. Add 6 new tokens to give apps full control over all visual elements.

New tokens

Token	Purpose	Currently hacked with
Surface	Panel/container background	nothing (transparent)
BorderFocused	Active/focused panel border	Primary or PrimaryBright
TitleText	Headings, section titles	Primary or Info (inconsistent)
TextAccent	Secondary emphasis text (links, labels)	Primary or Border (wrong semantic)
Highlight	Selected row/item background	PrimaryDim (semantic mismatch)
PopoverBorder	Popover overlay border	ModalBorder (reuse)

Changes required

1. Add the 6 new fields to style.Theme in style/theme.go
2. Update all 7 presets in style/presets.go with appropriate values
3. Update tuishell components to use the new tokens:
   • table/styles.go: use TitleText for TitleStyle, Highlight for selected row background
   • popover/*.go: use PopoverBorder instead of ModalBorder
   • shell/update.go / layout.go: use BorderFocused where applicable
4. Update example app theme

Source

• Roadmap section: Features
• Priority: medium
• Module: tuishell