feat: replace token counter with memory indicator widget

[NakataCode]

Replaces the static token-counter pill in the input footer with a
MemoryIndicator component — a circular arc gauge with a percentage
label that opens a popover on click. The popover shows a progress bar,
token detail, a brief explainer, and a "Start Fresh" button wired to
clearHistory.

• Adds MemoryIndicator.js and MemoryIndicator.less (new component)

• Removes .input-footer / .token-counter styles; introduces
  .memory-indicator-host, .memory-indicator-btn, .memory-popover

• AIChat now owns a _memoryIndicator instance; _updateTokenCounter
  becomes _updateMemoryIndicator, delegating all rendering to the widget

• Adds _reenableInput() helper so quota-exhausted disable is always
  paired with a clean re-enable path (clear, null usage, or sub-100%)

• Updates warning copy to reference "conversation memory" instead of
  "token limit"

• Extends AIChat.spec.js and adds MemoryIndicator.spec.js / integration
  suite covering hide/show gating, quota exhaustion, destroy-race guard,
  and null-usage re-enable

  JIRA: BGSOFUIPIRIN-7101